Specific Aim: To characterize the effect of Ritalin (methylphenidate) on neural activity underlying performance on the response inhibition task in ADHD adolescents and adults. Hypothesis 1) Methylphendiate will increase N2 and P3 amplitude in ADHD persons during medicated EEG sessions; Hypothesis 2) There will be a significant age × medication interaction such that ADHD teens will show increased amplitude of N2 while medicated, particularly at frontal sites, whereas ADHD adults will show differentially greater effect of medication on P3 amplitude and latency at central sites. Hypothesis 3) Brain activity assessed by fMRI will differ between unmedicated and medicated states.